Experimental investigation of the interaction between laser radiation and a target in air

An investigation was made of the interaction of laser pulses of 1 μsec duration and up to 30 J energy with a target in air on which the laser radiation focused in a spot of 5–8 mm diameter. The shape of a plasma jet formed on the target, velocity of the plasma front and shock wave, brightness temperature, and pressure were determined at different moments. The experiments were carried out in the power density range from 10 to 300 MW/cm² and the illumination in the focal spot was almost uniform. The results obtained indicated that up to about 100 MW/cm² an absorption wave formed in the leading edge of an opaque (for the incident radiation) jet and this wave was preceded by a transparent shock wave. The expanding plasma jet acted as a piston on the surrounding cold layers ("expulsion regime"). At higher power densities a detonation regime was observed and it was complicated by radiative energy transfer at wavelengths corresponding to the continuous spectrum of the hot plasma. During the initial part of a radiation pulse the flow conditions were nearly planar but as the jet rose above the target to distances of the order of illuminated spot size, the motion became two-dimensional.